Observed changes in snow depth and number of snow days in the eastern and central Tibetan Plateau

The Tibetan Plateau (TP) has the largest area of snow in the mid-latitude regions, and is strongly affected by the climate change. We examine the temporal variability of winter snow depth and number of days of snow cover at 69 Chinese Meteorological Administration stations above 2000 m a.s.l. in the eastern and central TP during 1961-2005. Snow depth is positively correlated with the number of snow days (R = 0.89, p < 0.0001). Regional mean winter (DJF) depth and days of snow cover increase at rates of 0.32 mm decade(-1) and 0.40 d decade(-1) from 1961 to 1990, but at rates of -1.80 mm decade(-1) and -1.59 d decade(-1) (i.e. decrease) between 1991 and 2005. The long term trends are weakly positive, but unrepresentative of shorter time periods. Thus snow depth and cover change depends on the timescale examined and cannot be attributed solely to increased greenhouse gas forcing. The decreasing snow depth in recent years will influence hydrological processes and water resources on the plateau and downstream. Both snow depth and duration have positive correlations with the winter Arctic Oscillation/North Atlantic Oscillation (AO/NAO) index and Nino-3 region (5 degrees N-5 S, 150 degrees-90 degrees W) sea surface temperature (SST). During high AO/NAO index years, both a deeper India-Burma trough and an intensified cyclonic circulation near Lake Baikal bring more snowfall to the TP, consistent with a higher water vapor flux. The opposite is true in low AO/NAO years. Thus secular changes of snow depth and duration in the TP are not independent of changes in the macro-scale atmospheric circulation